Hydrophilic polyurethane (HPU) foams are employed in a range of applications including medical applications. Medical HPU foams are commonly manufactured from an oligomeric urethane-ether pre-polymer, water and a surfactant (see Design and Applications of Hydrophilic Polyurethanes, T. Thomson, CRC Press, 2000). The urethane-ether prepolymer is commonly based upon one of two isocyanate pre-cursors: methylene diphenyl diisocyanate (MDI) or toluene diisocyanate (TDI). HPU foams are able to absorb many times their own weight in water-based liquids, including human serum, without significant visible change in appearance; this can make it difficult for a user or carer to assess how wet a HPU device is in situ in use. Whilst moist wound healing is the current paradigm for wound care, a wound dressing that is wet or moist can incubate bacterial growth and/or cause maceration of the periwound and this is detrimental to wound healing. Thus, there is a need to visualise the location and extent of moisture and liquid in HPU-based medical devices. This need is not restricted to HPU-based devices or medical devices.
Colour-formers or colour forming materials are chemicals that can exist in two or more coloured states, one of which may be colourless. The transition between coloured states is achieved by a change in the environment of the colour-former (CF); this can, for example, be a change in pressure, temperature, light intensity or solvent. In each of these cases, the CF requires custom formulation with other excipients to respond to each specific environmental change. In general, different excipients are required to translate different environmental stimuli into a change in the coloured state of the CF. Thus, the same CF can be employed in a range of colour-indicating applications for a variety of environmental changes.
The prior art includes materials which take on the colour of a fluid to which they are exposed, see for example WO2011/049522. These materials are not examples of a colour-forming material but, rather, are coloured, absorbent materials.
Known thermochromic CF systems typically rely upon a highly specific set of excipients. The excipient-specificity of one such system is investigated and demonstrated by White et al in Journal of Materials Science, 2005, 40, 669.
Moisture-indicating CF systems are reported in the prior art but they are unsatisfactory for a number of reasons, primarily because one or more of the components of each system is water-soluble; thus the indicating system can be washed-out from its intended location. This is unsuitable for most medical and household applications.
U.S. Pat. No. 7,913,640 discloses a system for indicating the presence of moisture (but not humidity) in heat and moisture exchange (HME) devices. The indicating system comprises an intimate mixture of CF and an activator that, in the absence of moisture, is colourless but, when exposed to moisture, becomes coloured to an extent that can easily be observed by eye. An example of an indicating system is a mixture of the CF crystal violet lactone and the activator 2-sulfosalicylic acid. The presence of moisture solubilises the activator and allows it to interact with the CF, resulting in colour formation.
US Patent Publication No. 2011/0144603 discloses a wetness indicating system for use in a range of absorbent devices. The system described relies upon four components: a leuco dye (CF), a colour developer (activator), a separator and an encapsulation matrix. The system is colourless in the dry state and becomes strongly coloured when wet due to the dissolution of the separator. Once again, this is undesirable for direct medical usage. The examples disclosed in US Patent Publication No. 2011/0144603 become coloured when wet and remain coloured when re-dried due to wash-out of the separator.
The minimum requirement for colour formation in any of the systems described above is a CF and an activator. For example, Ichimura et al describe the solid-state adsorption of crystal violet lactone (CF) on silica nanoparticles (activator) to generate strongly coloured products in Langmuir, 2008, 24(13), 6470-6479; demonstrating that a solvent is not a pre-requisite for such interactions.
The colour former crystal violet lactone is applied in a range of colour-indicating applications that rely upon its property of reversible colour change. The compound exists in its isolated pure state as an off-white lactone. In the presence of selected molecules or surfaces of appropriate charge or structure (activators), the lactone form of the molecule transforms to its charged, highly coloured leuco form.
GB2410748 and U.S. Pat. No. 3,635,652 discuss that polyurethane-based materials can be loaded with therapeutic agents or dyestuffs respectively. None of these agents or dyestuffs are colour forming species. U.S. Pat. No. 5,399,609 teaches that water-soluble moisture indicating particles (transition metal salts) can be mixed with thermoplastic polyurethane resins to indicate the moisture level in the material. The metal salts are immiscible with the polyurethane resin and remain particulate. Metal salts are not examples of colour forming materials.
US2012/0143160 discusses that colour forming systems comprising multiple components, at least some of which are water soluble, can be rendered less water soluble by entrapment in a water insoluble but water-permeable matrix.
Moreover, the inventor is not aware of prior art that discloses or anticipates the molecular dispersion or partial molecular dispersion of a colour former in a hydrophilic polyurethane for the indication of wetness.
It is an aim of the present invention to at least partly mitigate the problems associated with the prior art.
It is an aim of certain embodiments of the present invention to create a moisture-indicating system which is suitable for use in topical or penetrating medical devices or other non-medical devices.
It is an aim of certain embodiments of the present invention to provide a solid dispersion of a colour-forming material in a hydrophilic polyurethane for use to indicate wetness e.g. moisture.
It is an aim of certain embodiments of the present invention, to provide a system wherein none of the interacting species (e.g. pre-colorant, activator, solvent or separator) are solubilised when moisture is absorbed, thereby eliminating the risk of release of any of these species into the local environment of the device.
It is an aim of certain embodiments to provide a molecular dispersion or partial molecular dispersion of a colour-forming material in a hydrophilic polyurethane.
Furthermore, it is an aim of certain embodiments of the present invention to generate a moisture-indicating system that responds in a proportional colour-intensity manner to moisture level. In contrast, the prior art generate a step-change colour response.
It is an aim of certain embodiments of the present invention to generate a moisture-indicating system that is capable of reversible moisture indication.
Furthermore, it is an aim of certain embodiments of the present invention to generate a moisture-indicating system that is substantially colour-stable in moist and wet environments for periods of at least 24 hours.
Furthermore, it is an aim of certain embodiments of the present invention to generate a moisture-indicating system that is substantially stable following sterilisation.
Furthermore, it is an aim of certain embodiments of the present invention to generate a moisture-indicating system that is substantially light-stable.